There are a number of different known methods by which relatively large surface areas of an object can be simultaneously measured along x, y and z coordinates by projecting a regular, grating-shaped pattern onto the surface of the object. These methods, which are referred to as "moire topography", include different variants whereby the so-called moire effect (created by light which passes twice through a grating having the same period) is utilized to obtain height information pertaining to the object being measured.
One of these variants is the "shadow pattern" moire method described by Takasaki in Applied Optics 6 (1970), page 1467, in which the object surface to be tested is illuminated by point-source light that is passed through a grating positioned relatively close to the surface. The surface of the object is then viewed through the same grating, but from a point distant from the source of the measuring light, so that the illumination rays and the reflected imaging rays subtend an angle. Inasmuch as the grating pattern projected on the object surface is deformed as a function of the topography of said surface, contour lines are formed as a result of the moire effect as the imaging rays pass through the same grating; and these contour lines provide surface height information. With this method, the contour lines remain visible even if the basic frequency of the grating used for illumination is not resolved at the time of imaging, or even if the grating is "averaged away" by being shifted by one or more full grating periods during the recording of an image.
Another variant of the moire topography method is the so-called "projection moire" method described in U.S. Pat. No. 4,564,295. According to this method, an image of a grating is projected on the object surface, and an image of the object surface is then projected through a lens and a second grating positioned in front of the recording camera. This prior art method permits synchronous shifting of the two gratings--i.e., the projection grating and the grating used for imaging--during the imaging operation, thereby averaging out grating irregularities without changing the contour lines resulting from the moire effect or their location in space. However, this method requires that the gratings have the same grating constant and that the focal lengths of the projecting unit and the imaging lens be the same. This prior art patent further discloses that two projectors can be positioned symmetrically at the same distance and the same angle of projection on either side of the path of measuring rays, i.e., the camera axis. This double projection arrangement generates overlapping and adjoining grating patterns, thereby eliminating the problem of shadows when measuring severely curved object surfaces.
A third variant of the moire topography method dispenses with the second grating at the front of the recording camera and, instead, uses the line raster of the recording video camera or the pixel period of a CCD camera for the function of the second grating. This so-called "scanning moire" method is described in Applied Optics, Volume 16, No. 8 (1977), page 2152.
In addition to these just-described moire topography methods, it is also known that an object can be measured by calculating height information directly from the deformation of a bar grating pattern on the object surface without using a second grating in front of the camera. These so-called "bar projection methods" are described in European Patent No. EP-A2-0 262 089, and in U.S. Pat. Nos. 4,641,972; 4,488,172; and 4,499,492.
These prior art moire topography methods and bar projection methods produce quantitative coordinate measurement information by evaluating the cyclical brightness variations of the resulting bar patterns or contour lines. Generally, this is called phase measurement; and this is usually carried out in such a manner that, during the process of making each measurement, the position of the projection grating is shifted in several steps by fixed increments, often corresponding to a phase shift of 90.degree. or 120.degree..
However, with these known methods, it is relatively difficult to obtain adequate measurements of larger industrial objects, particularly if the surfaces of such objects have irregularities such as edges and tiers. The difficulty arises for the following reason:
Due to the symmetrical perspective arrangement of the projection and viewing rays, the distance of successive contour lines is not constant, but increases with the increasing depth of the object; and without the knowledge of the actual distance of the object surface from the camera at at least one point, it is not possible to obtain data pertaining to the form of a profile of a continuous surface. That is, such conventional methods of phase measurement calculate the object distance only in terms of one complete change of phase and, thus, provide only relative values within one ordinal number of a moire pattern. Therefore, it is not possible with these methods of phase measurement to accurately analyze the sudden large changes in the cyclical bar patterns that occur over the edges and tiers of irregular industrial objects.
Another problem relating to the use of said prior art measuring methods arises from the fact that these methods require or permit the bars projected on the surface being measured to be moved, resulting in a loss of reference between the bar or contour lines and the coordinate system fixed relative to the apparatus. While it is still possible to connect the measurements made with the different positions of the moving projection grating with high accuracy, this necessitates that the location of the gratings in each position be known absolutely within fractions of one grating period. Therefore, this requires that each shift of the grating must either be measured with great accuracy (which is possible only with relatively great effort and expense) or that the positions between which shifting occurs be maintained as precisely as possible. Unfortunately, this latter requirement is not easily accomplished, because even minor changes in the position of the surface to be measured relative to the measuring instrument, e.g., such as those resulting from minor vibrations, will result in uncontrollable phase changes which will affect the measured result.
Further, it should be noted that when accurate resolution of objects having great height variations is desired, the focus of the camera must be adjusted for various depth ranges. However, this cannot be readily accomplished with prior art systems because, when the focus is moved, it is inevitably accompanied by small lateral shifts of the image of the object being measured relative to the projection grating or the camera. Such shifts change the measured phase relationships of the bar lines or contour lines; and therefore, error-free measurements can no longer be obtained when adjustments in focus settings are required.
Still further, the prior art methods are relatively slow, because accurate phase measurement requires the recording of at least three sequential images of the shifting bar pattern.
The present invention overcomes these problems by providing a moire-type measuring method and apparatus characterized by high accuracy in which the measurement is quite insensitive to interfering environmental influences.